Digester Forum Presents Ideas for Small-Scale Production

LANCASTER, Pa. — Manure digesters are thought of highly in the Keystone State.

State environmental officials hope to use them as a tool to reduce the amount of nitrogen and phosphorus that eventually flows into the Chesapeake Bay.

But for all of the news one manure digester makes here when it is built, the technology has been around for well over 100 years in other parts of the world.

In fact, small-scale digesters number in the millions.

Stephanie Lansing, assistant professor in the department of environmental science and technology at the University of Maryland, thinks there is a growing market for small-scale digesters in the U.S.

She was one of several speakers at an anaerobic digester workshop at the Lancaster Farm and Home Center last Thursday.

About 150 people attended the meeting, which was put on by Penn State Cooperative Extension and AgSTAR, a joint program of the Environmental Protection Agency (EPA) and USDA, whose main purpose is to provide resources for manure digester development.

The average manure digester, Lansing said, is designed to operate on a farm with at least 500 cows, largely because of cost, economy of scale and the proliferation of nutrient credit markets.

Around 160 of these anaerobic digesters are currently in operation in the U.S. But that pales in comparison with the rest of the world, where 20,000 are in operation.

The number is even lower for small-scale digesters. She knows of only 13 in operation in the U.S., while China alone has 37 million of them.

While most manure digesters currently on farms or in development rely on the latest technology, most of the digesters Lansing has seen in other countries are anything but advanced.

They are literally small pits in a backyard that are covered in plastic and take advantage of the natural heat to create bacteria that break down nutrients to produce biogas.

Subsistence landowners, who rely on a few cows or pigs to live, use biogas from the digester for heating, cooling and cooking.

Most of what goes into a digester is animal waste, but it’s not the only thing. Human waste, food waste and other things are used in these fairly crude systems.

And they are cheap to install. In Costa Rica, where Lansing visited farms to see how they were being used, the cost of putting in a digester ranges from $150 to $1,500.

There is not much needed, besides some plastic and other things to make a system work.

“We are looking at cheap production here,” Lansing said.

Now Lansing wants to see just how easy or difficult it could be to develop these digesters in a colder climate like Maryland or Pennsylvania.

She is currently involved in a research study at the USDA’s Beltsville Agricultural Research Center on small-scale anaerobic digestion.

The key to making a system like this work around here, she said, is doing it in cold weather.

“When you digest in cold climates, you need to maintain the internal temperature,” she said.

In tropical climates, a landowner can take advantage of the warm weather to keep the digester pit nice and warm for bacteria to grow. The study is looking at ways of insulating systems to keep in the hot air so production can continue in the winter.

The study is also focusing on digester models that are much more energy efficient. Some problems she has seen around the world include accumulation of solids in the system and systems themselves breaking down after only a few years. Many landowners just don’t have the know-how to deal with potential problems or issues that arise, so many broken down digesters are abandoned.

Manufacturers, she said, have started developing pre-fab models that are more technologically advanced.

If anything, Lansing is hoping to prove that small-scale manure digestion is possible, even on small dairies.

“You don’t need 1,000 cows to produce biogas. We’re trying to take tropical systems and bring them here,” she said. “It is definitely a growing market. The market is potentially very good for this.”

Robert Graves, an agricultural engineer at Penn State, said a digester must maintain an internal temperature of 100 degrees Fahrenheit to help support the “bugs” that feed off manure and produce biogas.

The actual biogas, he said, doesn’t produce much energy. One cubic foot of biogas can produce around 600 Btu of energy, which he said pales in comparison with oil or natural gas.

In terms of the removal of actual manure, it doesn’t get rid of much, so a manure storage area will still be needed.